1. Field of the Invention
The present invention relates to a paper sheet discriminating device which discriminates paper sheets such as bills, stamps, checks, drafts, gift certificates and the like, and more particularly to a paper sheet discriminating device which irradiates lights of at least two wavelengths to the paper sheet and performs the discrimination in response to light receiving signals of transmitting light through the paper sheet and prevents the lowering of the discrimination accuracy due to the irregularities of the outputs of the photo sensor.
2. Description of the Related Art
A conventional paper sheet discriminating device for discriminating paper sheets includes a light source which alternately irradiates lights of two wavelengths (for example, red light and infrared light) to a bill, a photo sensor which receives transmitting lights through a bill as lights intrinsic to respective wavelengths, and a processing discrimination circuit which processes received light signals from the photo sensor and performs the discrimination. In the processing discrimination circuit, since the bill is discriminated by performing the relative evaluation of received light output values between two wavelengths, the received light output levels of the transmitting lights through the bill must be held at given levels at respective wavelengths.
To this end, conventionally, at the time of adjusting the light emission quantity of the light source, first of all, the light emission quantity of the light source is adjusted such that the output values of the photo sensor which receives the direct light from the light source becomes given values. Subsequently, a reference medium is set between the light source and the photo sensor, the ratio of read values (output values) of the photo sensor to the target values (target values/read values) is calculated, and new values obtained by multiplying the ratio to the current given values are stored as light receiving adjustment values. Thus the adjustment of the sensor completes.
FIG. 1 shows the conventional sensor adjustment method using no correction coefficient. First of all, the light quantity is adjusted to the adjustment target value (fixed value) in the state that no bill exists in a passage and thereafter the discrimination of the bill is performed based on the photo sensor output in response to the transporting bill. In such an adjustment method, however, due to the irregularities of the directivity of the light receiving element of the photo sensor, the directivity of the light emitting diode of the light source, the mounting angle and the mounting position of the light receiving element and the light emitting diode, the distance between sensors, the bill passing positions or the like, the characteristics vary in every device. Accordingly, when the output is taken by the photo sensor while transporting the bill, as can be understood from the sensor output in the xe2x80x9cbill presentxe2x80x9d state shown in FIG. 1, the sensor output varies depending on the characteristics curves. This can be said with respect to respective lights of two-wavelength light.
FIGS. 2 and 3 also show the conventional sensor adjustment method. These examples relate to cases where the correction coefficients are stored for respective devices. First of all, the storing procedure of the correction coefficient at the time of shipping is performed as shown in FIG. 2. That is, in the state that no bill is present in a passage, the light quantity is adjusted to the target value A, a white reference medium is set on the photo sensor, and an adjustment target value D of every time is obtained in accordance with a following equation (1) based on an output value B at the point of time and an output value C of the reference device (central device) and is stored in a memory.
D=Axc3x97C÷Bxe2x80x83xe2x80x83(1) 
Subsequently, the adjustment for every judgement at the point of time of discrimination is performed in accordance with FIG. 3. First of all, the light quantity is adjusted such that the light quantity becomes the target value D stored in the memory in the state that no bill exists in the passage, and thereafter, in accordance with the characteristics at the point of time (solid line in FIG. 3), the judgement is performed based on the sensor output in response to the transporting bill. However, in this case, as indicated by the xe2x80x9creference medium presentxe2x80x9d state in FIG. 3, it also gives rise to the difference in the sensor output (Bxe2x80x2, Cxe2x80x2) between the characteristics of the reference device and the characteristics of the actually operating device (broken line). This can be said with respect to respective lights of two-wavelength light source.
As mentioned above, since there exists the irregularities in every device in case the sensor adjustment method has no correction coefficient, there has been a drawback that the sensor output differs in every device. Further, in case of storing the correction coefficient of every device, the light emission quantity of the light source whose characteristics are not linear is adjusted by preliminarily determined computing values. Accordingly, due to the irregularities of the characteristics of the light source which differ in every wavelength and the characteristics of the photo sensor, the mounting error of the light source and the photo sensor, the fluctuation of temperature, the change which occurs as time lapses, the irregularities of circuits or the like, a given output level of the photo sensor with respect to the reference medium varies due to the difference of device. Further, it also gives rise to the difference in the output level of the photo sensor between two wavelengths. Accordingly, the highly accurate discrimination (detection of forged paper sheet) using the received-light output value between the two-wavelengths has been difficult.
The present invention has been made in view of the above and it is an object of the present invention to provide a paper sheet discriminating device which can perform the highly accurate paper sheet discrimination by making respective output levels of a photo sensor for lights of at least two or more wavelengths agree with each other and reducing the irregularities of the output levels of the photo sensor even at individual paper sheet discrimination parts.
The present invention is directed to a paper sheet discriminating device which irradiates lights of at least two or more wavelengths from a light source to a paper sheet and receives transmitting lights which transmit through the paper sheet by a photo sensor and performs the discrimination of the paper sheet in response to light receiving signals from the photo sensor, wherein the above-mentioned object of the present invention is achieved by providing reference value setting means which adjusts a light emission quantity of the light source such that the output of the photo sensor becomes a given value in a state that a reference medium is set between the light source and the photo sensor and also stores the output value of the photo sensor which directly receives light from the light source as an adjustment reference value, and adjustment means which adjusts the light emission quantity of the light source such that the output value of the photo sensor which directly receives light from the light source is made to agree with the stored adjustment reference value.
Further, the above-mentioned object of the present invention is more effectively achieved by commonly using the lights of at least two or more wavelengths as the light source, by providing a light receiving circuit system which is comprised of an amplifying circuit for amplifying output signals from the photo sensor, a gain changeover circuit which changes over output gains, an offset circuit which adjusts an offset and a separation circuit for separating output signals of lights of respective wavelengths, by arranging the light source and the photo sensor in an opposed manner, by making the paper sheet transported between the light source and the photo sensor, providing a pressing mechanism which presses the paper sheet to the light source side to the photo sensor side, and including any one of infrared light, red light and blue light in the lights of at least two wavelengths.
To be more specific, at the time of initially setting the light receiving adjustment reference value of the two-wavelength light source, for example, the white reference medium is set between the two-wavelength light source and the photo sensor and the light emission quantity of the two-wavelength light source is adjusted such that the output of the photo sensor which receives the transmitting light through the reference medium becomes a given value. Then, with this adjusted light emission quantity, an output value of the photo sensor which directly receives light from the light source in the state that the reference medium is removed is stored in a memory as the light receiving adjustment reference value. Such a setting operation is performed sequentially with respect to two wavelengths. Then, right before starting the discrimination, the light emission quantity of the two-wavelength light source is automatically adjusted. This automatic adjustment is performed in the state that all drive mechanisms are stopped so as to eliminate the influence of noises. Further, since the light emission quantity of the two-wavelength light source is adjusted such that the output of the photo sensor which directly receives light is made to agree with the light receiving adjustment reference value stored as the reference value, the output of the photo sensor in response to the transmitting light through the paper sheet becomes the given level at the time of initial setting with respect to both of the two wavelengths whereby the irregularities of the output levels of the photo sensor between two wavelengths can be suppressed.
The paper sheet discriminating device of the present invention alternately turns on lights of two wavelengths from the two-wavelength light source and irradiates the lights to the paper sheet, detects the transmitting lights through the paper sheet with the photo sensor, and performs the judgment of truth or false of the paper sheet in response to detected signals. A diffusion plate is arranged between the two-wavelength light source and the photo sensor and reduces the influence of irregularities of the directivity, the mounting angle and mounting distance of the two-wavelength light source. Further, the photo sensor and the light receiving circuit system for the two wavelength lights are constituted such that they can be formed into a single unit and is commonly used to the two wavelengths and the output signal of the photo sensor is separated into two wavelengths finally and hence, the offset of the output of the photo sensor between two wavelengths derived from the irregularities of the photo sensor or the circuit depending on the devices can be reduced. Further, although a distance for allowing the passing of the paper sheet (transport passage) is formed between the two-wavelength light source and the photo sensor, the paper sheet is pressed to a guide arranged at the two-wavelength light source side by transport belts which are arranged at both sides of the photo sensor so as to make the sensor passing position of the paper sheet (distance between the paper sheet and the photo sensor) constant whereby the irregularities of the output of the photo sensor due to the sensor passing position of the paper sheet can be suppressed.
Due to such a constitution, the output level of the photo sensor to the lights of two wavelengths becomes stable so that the highly accurate discrimination of the paper sheet becomes possible whereby the detection ability of forged paper sheets is enhanced. Further, by adding the blue light to the infrared light and the red light which constitute the lights of two wavelengths in the light source, the detection ability particularly to copied certificates can be enhanced.